Abstract. This proposal is focused on investigation of hepatitis C virus (HCV) immune evasion mechanisms and protective immunity. Strategies to eradicate chronic viral infections include vaccination and delivery of curative therapies for persistent infection, but these have been challenging for hepatitis B virus, human immunodeficiency virus, and HCV. While most viral infections induce sterilizing T cell responses, these viruses have evolved immune evasion mechanisms that allow viral persistence. HCV is an ideal infection in which to study viral evasion mechanisms because infection persists in the majority of infected individuals, while roughly 25% clear the virus. We have recently demonstrated that HCV evades CD8 T cell responses in part by mutating epitopes, and there are several mechanisms through which mutations in CD8 T cell epitopes enable escape. Additional potential strategies for evasion of CD8 T cell responses include activation of T cell inhibitory molecules and failure to generate or maintain effector/memory T cells. Our central hypothesis is that both immune escape and evasion mechanisms affect the balance between successful viral control and progression to chronicity. Consequently, the ultimate goal of this proposal is to ask how mechanisms of HCV immune evasion and immune regulation determine the quantitative and qualitative nature of HCV-specific CD8 T cell responses and ultimately, correlate with the outcome of infection as a model of viral persistence. Specifically, we plan to 1) compare the kinetics, frequency, and type of escape mutations in CD8 T cell epitopes in the hyper-acute phase of infection and re- infection among individuals who clear HCV infection and those who progress to chronic infection 2) test the hypothesis that expression of PD-1 and CD127 affects the function of CD8 T cells specific for non-mutated HCV epitopes and evaluate mechanisms of regulation of their expression and 3) determine the role of PD-1 and CD127 in the response of memory T cells to repeated HCV exposure. We have already demonstrated that we can obtain the critical samples required in this investigation, conduct unique longitudinal studies of persons from the time of initial infection, and measure cellular immune responses. These unique patient resources, combined with viral sequence evolution expertise and immunologic expertise, assures the feasibility of the aims embodied within this proposal. These insights may increase understanding of human chronic viral infections and be useful in the development of immunotherapies.